[Repository] refactor helm to use managed-by annotation instead of system flag

[dlaloue-vmware]

For repositories, secrets used for authentication can be either user managed or kubeapps managed. The current mechanism is based on a system flag to enable one more or the other mode. This limits the flexibility to have some users use one way (e.g. cli) or the other (e.g. UI) in a single Kubeapps installation.

this issue is to refactor the helm plugin.